Metathesis reactions are used widely in chemical syntheses, e.g. in ring-closing metathesis (RCM), cross-metathesis (CM), ring-opening metathesis (ROM), ring-opening metathesis polymerizations (ROMP), cyclic diene metathesis polymerizations (ADMET), self-metathesis, reaction of alkenes with alkynes (enyne reactions), polymerization of alkynes and olefinization of carbonyls (WO-A-97/06185 and Platinum Metals Rev., 2005, 49(3), 123-137). Metathesis reactions are employed, for example, for the synthesis of olefins, for the ring-opening polymerization of norbornene derivatives, for the depolymerization of unsaturated polymers and for the synthesis of telechelic polymers.
Metathesis catalysts are known, inter alia, from WO-A-96/04289 and WO-A-97/06185. They have the following basic structure:
where M is osmium or ruthenium, the radicals R are identical or different organic radicals having a wide range of structures, X1 and X2 are anionic ligands and L is in each case an uncharged electron donor. The customary term “anionic ligands” is always used in the literature concerning such metathesis catalysts to describe ligands which, when viewed as removed from the metal centre, are negatively charged when they have a closed electron shell.
Metathesis reactions have recently also become increasingly important for the degradation of nitrile rubbers.
The term nitrile rubber, also referred to as “NBR” for short, refers to rubbers which are copolymers or terpolymeres of at least one α,β-unsaturated nitrile, at least one conjugated diene and, if appropriate, one or more further copolymerizable monomers.
Hydrated nitrile rubber, also referred to as “HNBR” for short, is produced by hydration of nitrile rubber. Accordingly, the C═C double bonds of the copolymerized diene units are completely or partly hydrated in HNBR. The degree of hydration of the copolymerized diene units is usually in the range from 50 to 100%. Hydrated nitrile rubber is a speciality rubber which has very good heat resistance, excellent resistance to ozone and chemicals and also an excellent oil resistance.
The abovementioned physical and chemical properties of HNBR are combined with very good mechanical properties, in particular a high abrasion resistance. For this reason, HNBR has found wide use in a wide variety of applications. HNBR is used, for example, for seals, hoses, belts and damping elements in the automobile sector, also for stators, well seals and valve seals in the field of oil production and also for numerous parts in the aircraft industry, the electrical industry, machine construction and shipbuilding.
Most commercially available HNBR grades usually have a Mooney viscosity (ML 1+4 at 100° C.) in the range from 55 to 120, which corresponds to a number average molecular weight Mn (determination method: gel permeation chromatography (GPC) against polystyrene standards) in the range from about 200 000 to 700 000. The polydispersity indices PDI (PDI=MW/Mn, where Mw is the weight average molecular weight and Mn is the number average molecular weight), which indicate the width of the molecular weight distribution, measured here frequently have a value of 3 or above. The residual double bond content is usually in the range from 0 to 18% (determined by NMR or IR spectroscopy). However, the term “fully hydrated grades” is used in the technical field when the residual double bond content is not more than about 0.9%.
The processability of HNBR grades having the abovementioned relatively high Mooney viscosities is subject to limitations. For many applications, it is desirable to have HNBR grades which have a lower molecular weight and thus a lower Mooney viscosity, since this significantly improves the processability.
Many attempts have been made in the past to shorten the chain length of HNBR by degradation. For example, it is possible to reduce the molecular weight by thermomechanical treatment (mastication), e.g. on a roll mill or also in a screw apparatus (EP A-0 419 952). However, this thermomechanical degradation has the disadvantage that functional groups such as hydroxyl, keto, carboxylic acid and ester groups are built into the molecule as a result of partial oxidation and, in addition, the microstructure of the polymer is substantially altered.
The preparation of HNBR having low molar masses, corresponding to a Mooney viscosity (ML 1+4 at 100° C.) in the range below 55 or a number average molecular weight Mn of <200 000 g/mol, has for a long time not been possible by means of established production processes since, firstly, a step increase in the Mooney viscosity occurs in the hydration of NBR and, secondly, the molar mass of the NBR feedstock used for the hydration cannot be reduced at will since otherwise processing in the available industrial plants is no longer possible because of excessive high stickiness. The lowest Mooney viscosity of an NBR feedstock which can be processed without difficulties in an established industrial plant is about 30 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity of the hydrated nitrile rubber obtained from such an NBR feedstock is in the order of 55 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity is determined in accordance with ASTM Standard D 1646.
In the more recent prior art, this problem is solved by reducing the molecular weight of the nitrile rubber by degradation to a Mooney viscosity (ML 1+4 at 100° C.) of less than 30 Mooney units or a number average molecular weight Mn of <70 000 g/mol before hydration. The decrease in the molecular weight is achieved by metathesis in which low molecular weight 1-olefins are usually added. The metathesis of nitrile rubber is described, for example, in WO-A-02/100905, WO-A-02/100941 and WO-A-03/002613. The metathesis reaction is advantageously carried out in the same solvent as the hydration reaction so that the degraded nitrile rubber does not have to be isolated from the solvent after the degradation reaction is complete before it is subjected to the subsequent hydration. The metathesis degradation reaction is catalysed using metathesis catalysts which are tolerant to polar groups, in particular nitrile groups.
WO-A-02/100905 and WO-A-02/100941 describe a process which comprises the degradation of nitrile rubber starting polymers by olefin metathesis and subsequent hydration to give HNBR having a low Mooney viscosity. Here, a nitrile rubber is reacted in a first step in the presence of a coolefin and specific complex catalysts based on osmium, ruthenium, molybdenum or tungsten and hydrated in a second step. In this way, it is possible to obtain hydrated nitrile rubbers having a weight average molecular weight (Mw) in the range from 30 000 to 250 000, a Mooney viscosity (ML 1+4 at 100° C.) in the range from 3 to 50 and a polydispersity index PDI of less than 2.5.
For the metathesis of nitrile rubber, it is possible to use, for example, the catalyst bis(tricyclohexylphosphine)benzylidene ruthenium dichloride shown below.

After metathesis and hydration, the nitrile rubbers have a lower molecular weight and a narrower molecular weight distribution than the hydrated nitrile rubbers which could hitherto be prepared according to the prior art.
However, the amounts of Grubb (I) catalyst employed for carrying out the metathesis are large. In the experiments in WO-A-03/002613, they are, for example, 307 ppm and 61 ppm of Ru based on the nitrile rubber used. The reaction times necessary are also long and the molecular weights after degradation are always still relatively high (see Example 3 of WO-A-03/002613, in which Mw=180 000 g/mol and Mn=71 000 g/mol).
US 2004/0127647 A1 describes blends based on low molecular weight HNBR rubbers having a bimodal or multimodal molecular weight distribution and vulcanizates of these rubbers. According to the examples, the metathesis is carried out using 0.5 phr of Grubbs I catalyst. This corresponds to an amount of 614 ppm of ruthenium based on the nitrile rubber used.
Furthermore, WO-A-00/71554 discloses a group of catalysts which are referred to in the art as “Grubbs (II) catalysts”.
If such a “Grubbs (II) catalyst”, e.g. the catalyst 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidenylidene)(tricyclohexylphosphine)(phenylmethylene)ruthenium dichloride shown below, is used for the NBR metathesis (US-A-2004/0132891), this can be carried out successfully without use of a coolefin.

After the subsequent hydration, which is preferably carried out in the same solvent, the hydrated nitrile rubber has lower molecular weights and a narrower molecular weight distribution (PDI) than when catalysts of the Grubbs (I) type are used. The metathetic degradation using catalysts of the Grubbs (II) type thus proceeds more efficiently in respect of the molecular weight and the molecular weight distribution than when using catalysts of the Grubbs (I) type. However, the amounts of ruthenium necessary for this efficient metathetic degradation are still relatively high. In addition, long reaction times are still required for carrying out the metathesis using the Grubbs (II) catalyst.
In all the abovementioned processes for the metathetic degradation of nitrile rubber, relatively large amounts of catalyst have to be used and long reaction times are required in order to prepare the desired low molecular weight nitrile rubbers by means of metathesis.
In the other types of metathesis reactions, too, the activity of the catalysts used is of critical importance.
In J. Am. Chem. Soc. 1997, 119, 3887-3897, it is stated that in the ring-closing metathesis of diethyl diallylmalonate shown below,
the activity of the catalysts of the Grubbs (I) type can be increased by additions of CuCl and CuCl2. This activity increase is explained by a shift in the dissociation equilibrium as a result of which a phosphane ligand which has been released reacts with copper ions to form copper-phosphane complexes.
However, this activity increase brought about by copper salts in the ring-closing metathesis mentioned cannot be applied at will to other types of metathesis reactions. Our own studies have shown that although the addition of copper salts leads to an initial acceleration of the metathetic degradation of nitrile rubbers, a significant worsening of the efficiency of the metathesis is then unexpectedly observed: the molecular weights of the degraded nitrile rubbers which are ultimately achieved are substantially higher than when the metathesis reaction is carried out in the presence of the same catalyst but in the absence of the copper salts.
EP-A-1 825 913 describes new catalyst systems for metathesis, in which not only the actual metathesis catalyst but also one or more salts are used. This combination of one or more salts with the metathesis catalyst leads to an increase in the activity of the catalyst, a synergistic effect. The anions and cations of these salts can have a large number of meanings which can be selected from various lists. In the examples of EP-A-1 825 913, the use of lithium bromide is found to be particularly advantageous both for the metathetic degradation of rubbers such as nitrile rubbers and also for the ring-closing metathesis of diethyl diallylmalonate. Catalysts mentioned are, in particular, ones which coordinate via an oxygen-, nitrogen- or sulphur-containing substituent to the metal centre of a ruthenium- or osmium-carbene. Use is made of, for example, the Grubbs (H) catalyst, the Hoveyda catalyst, the Buchmeiser-Nuyken catalyst and the Grela catalyst.
An as yet unpublished German patent application describes specific catalyst systems for metathesis, which comprise not only the actual metathesis catalyst but also alkaline earth metal chlorides, preferably magnesium or calcium chloride, as added salts.
EP-A-1 894 946 describes an increase in activity of metathesis catalysts as a result of specific phosphane additions.
The increase in activity of metathesis catalysts brought about by salts has likewise been studied in Inorganica Chimica Acta 359 (2006) 2910-2917. The influences of tin chloride, tin bromide, tin iodide, iron(II) chloride, iron(II) bromide, iron(III) chloride, cerium(III) chloride*7H2O, ytterbium(III) chloride, antimony trichloride, gallium dichloride and aluminium trichloride on the self-metathesis of 1-octene to form 7-tetradecene and ethylene were examined. When using the Grubbs (I) catalyst, a significant improvement in the conversion to 7-tetradecene was observed when tin chloride or tin bromide was added (Table 1; catalyst 1). Without addition of salt, a conversion of 25.8% was achieved; when 5 nCl2*2H2O was added, the conversion rose to 68.5% and when tin bromide was added it rose to 71.9%. Addition of tin iodide resulted in a significant decrease in the conversion from 25.8% to 4.1%. In combination with the Grubbs (II) catalyst (Table 1; catalyst 2), on the other hand, all three tin salts led to only slight improvements in yield from 76.3% (reference experiment without addition) to 78.1% (SnCl2), to 79.5% (SnBr2) and 77.6% (SnI2). When “phobcat” [Ru(phobCy)2Cl2(═ChPh)] is used (Table 1; catalyst 3), the conversion is decreased from 87.9% to 80.8% by addition of SnCl2, to 81.6% by SnBr2 and to 73.9% by SnI2. When iron(II) salts are used in combination with the Grubbs (I) catalyst (Table 3; catalyst 1), the increase in conversion when using iron(II) bromide is higher than when using iron(II) chloride. It may be remarked that, regardless of the type of catalyst used, the conversion when bromides are used is always higher than when the corresponding chlorides are used.
However, the use of tin or iron(II) bromide described in Inorganica Chimica Acta 359 (2006) 2910-2917 is not an optimal solution for the preparation of nitrile rubbers because of the corrosive nature of the bromides.
In the preparation of hydrated nitrile rubbers, the solvent is usually removed by steam distillation after the hydration. If tin salts are used as part of the catalyst system, amounts of these tin salts get into the wastewater which therefore has to be subjected to costly purification. For this reason, the use of tin salts for increasing the activity of catalysts in the preparation of nitrile rubbers is not advisable from an economic point of view.
The use of iron salts is restricted by the fact that they reduce the capacity of some ion-exchange resins which are usually employed for recovering the noble metal compounds used in the hydration. This likewise adversely affects the economics of the overall process.
ChemBioChem 2003, 4, 1229-1231 describes the synthesis of polymers by a ring-opening metathesis polymerization (ROMP) of norbornyl oligopeptides in the presence of a ruthenium-carbene complex Cl2(PCy3)2Ru═CHphenyl, with lithium chloride being added. The addition of lithium chloride is carried out with the declared objective of avoiding aggregation and increasing the solubility of the growing polymer chains. Nothing is said about an activity-increasing effect of the addition of the salt on the catalyst.
A method of carrying out a ring-opening polymerization of oligopeptide-substituted norbornenes is also known from J. Org. Chem. 2003, 68, 2020-2023, in which lithium chloride is used. Here too, the influence of lithium chloride as solubility-increasing additive for the peptides in nonpolar organic solvents is emphasized. For this reason, an increase in the degree of polymerization “DP” can be achieved by the addition of lithium chloride.
In J. Am. Chem. Soc. 1997, 119, 3887-3897 it is stated that when LiBr or NaI is added to a metathesis catalyst containing NHC ligands, e.g. the Grubbs (II) catalyst, the chloride ligands are replaced by bromide or iodide. Furthermore, it is shown that the catalyst activity depends on the type of halide ligands and increases in the following order: I<Br<Cl.
In J. Am. Chem. Soc. 1997, 119, 9130-9136 it is stated that the activity of the Grubbs (I) catalyst in the ring-closing metathesis of 1,ω-dienes can be increased by addition of tetraisopropoxytitanate and an improvement in yield can be achieved thereby. In the cyclization of the 9-decenoic ester of 4-pentenoate, a higher yield of the macrolide is achieved when tetraisopropanoxytitanate is added than when LiBr is added. There is no indication of the extent to which this effect can also be applied to other types of metathesis reactions or other metathesis catalysts.
In Org. Biomol. Chem. 2005, 3, 4139-4142 the cross-metathesis (CM) of acrylonitrile with itself and with other functional olefins when using [1,3-bis(2,6-dimethylphenyl)-4,5-dihydroimidazol-2-ylidene](C5H5N)2(Cl)2Ru═CHPh is examined. The yield of the respective product is improved by addition of tetraisopropoxytitanate. This publication gives the impression that the activity-increasing effect of tetraisopropoxytitanate occurs only when using a specific catalyst having pyridine ligands. There is no suggestion of an influence of tetraisopropoxytitanate when pyridine-free catalysts are used or in other types of metathesis reactions.
It is known from Synlett 2005, No. 4, 670-672 that the addition of tetraisopropoxytitanate in the cross-metathesis of allyl carbamate with methyl acrylate has an adverse effect on the product yield when the Hoveyda catalyst is used as catalyst. Thus, the product yield is reduced from 28% to 0% by addition of tetraisopropoxytitanate. Addition of dimethylaluminium chloride, too, reduces the yield from 28% to 20%.
In Synlett 2005, No. 4, 670-672 it is also stated that the product yield in the cross-metathesis of low molecular weight olefins is improved when specific boric acid derivatives are used. Use is made of chlorochatecholborane (ArO2BC1), dichlorophenylborane (PhBCl2) and chlorodicyclohexylborane (Cy2BCl). Depending on the boric acid derivative, the yield is improved to very different extents. To obtain appropriate improvements in yield, addition of 10-20 mol % of the boric acid derivative based on 1 equivalent of an olefin is required.
In Synthesis 2000, No. 12, 1766-1773 it is stated that the yields in the ring-closing metathesis of diethyl diallylmalonate when using the Grubbs I catalyst are not adversely affected by additions of boron trichloride and aluminium trichloride (Table 2). In a tandem enyne methathesis/Diels-Alder reaction of N-allyl-N-3-phenylprop-2-ynyl-p-toluenesulphenamide to form 4-acyl-7-phenyl-hexahydroisoindole via N-tosyl-1-(1-phenylvinyl)-2,4-dihydro-2H-pyrrole (as intermediate product of the enyne methathesis), too, the yield is not influenced by whether BCl3 is added right at the beginning together with the Grubbs (I) catalyst in a one-pot reaction or whether it is, in a sequential procedure, added only in the second step of the Diels-Alder reaction. These experiments show that the activity of the Grubbs (I) catalyst is not reduced by addition of boron trichloride or aluminium trichloride. However, there is no evidence that addition of boron trichloride or aluminium trichloride improves the catalyst activity.
Since the metathesis reaction is enjoying increasing popularity both in the field of low molecular weight chemistry and for polymers such as nitrile rubbers, there is, despite the existing prior art, a continuing need for improved catalyst systems for metathesis reactions and in particular the reduction of the molecular weight of nitrile rubber by metathesis. This applies all the more in view of the fact that, on the basis of the available prior art, results of one metathesis reaction cannot readily be applied to another.
In the light of this background, it is an object of the present invention to provide novel catalyst systems which can be used universally in various types of metathesis reactions, lead to activity increases for a wide variety of metathesis catalysts used as a basis and thus allow a reduction in the amount of catalyst and thus, in particular, the amounts of noble metal present therein. For the metathetic degradation of nitrile rubber in particular, possible ways of increasing the activity of the catalyst used without gelling of the nitrile rubber should be found.